When drilling for oil and gas, it is desirable to maintain maximum control over the drilling operation, even when the drilling operation may be several kilometers below the surface. Steerable drill bits can be used for directional drilling and are often used when drilling complex borehole trajectories that require accurate control of the path of the drill bit during the drilling operation.
Directional drilling is complicated because the steerable drill bit must operate in harsh borehole conditions. The steering mechanism is typically disposed near the drill bit, and the desired real-time directional control of the steering mechanism is remotely controlled from the surface. Regardless of its depth within the borehole, the steering mechanism must maintain the desired path and direction and must also maintain practical drilling speeds. Finally, the steering mechanism must reliably operate under exceptional heat, pressure, and vibration conditions that will typically be encountered during the drilling operation.
Many types of steering mechanism are used in the industry. A common type of steering mechanism has a motor disposed in a housing with a longitudinal axis that is offset or displaced from the axis of the borehole. The motor can be of a variety of types including electric and hydraulic. Hydraulic motors that operate using the circulating drilling fluid are commonly known as a “mud” motors.
The laterally offset motor housing, commonly referred to as a bent housing or “bent sub”, provides lateral displacement that can be used to change the trajectory of the borehole. By rotating the drill bit with the motor and simultaneously rotating the motor housing with the drillstring, the orientation of the housing offset continuously changes, and the path of the advancing borehole is maintained substantially parallel to the axis of the drillstring. By only rotating the drill bit with the motor without rotating the drillstring, the path of the borehole is deviated from the axis of the non-rotating drillstring in the direction of the offset on the bent housing.
Another steering mechanism is a rotary steerable tool that allows the drill bit to be moved in any chosen direction as the drillstring rotates. In this way, the direction (and degree) of curvature of the borehole can be controlled during the drilling operation, and can be chosen based on the measured drilling conditions at a particular borehole depth. Rotary steerable tools can be configured as point-the-bit or push-the-bit system to steer drilling.
Typically, the rotary steerable tool uses a reference of the tool's position while drilling so the tool can steer the advancing borehole in the correct direction. For the reference, the tool uses the angular position of the tool while drilling to steer the wellbore in the correct direction. Position readings are usually obtained from magnetometers and accelerometers on the tool to give the tool's angular position relative to the Earth's geomagnetic or gravitational fields. However, an angular offset (i.e., difference) between the gravitational and geomagnetic reference fields can vary depending on the orientation of the borehole. This angular offset may be referred to as the toolface offset (sometimes referenced as “TFO”). Typically, the toolface offset is calculated when the drilling assembly is stationary.
For example, the rotary steerable tool actively drilling ahead and using only a geomagnetic field or a gravitational field of reference may not need to account for this toolface offset. However, under some circumstances while drilling ahead, the rotary steerable tool may need to orient within the gravitational field of reference (i.e., towards a target gravity toolface) and yet operate internally using the geomagnetic field of reference. In this circumstance, the toolface offset provides the ability to translate the tool's target angularity between the two reference fields.
Because the rotary steerable tool rotates in the advancing borehole and steers ahead in a changing trajectory, a need exists to determine the toolface of the steerable tool and to account for any offset between the geomagnetic and gravitational fields of reference at regular intervals. Typically, these regular intervals are chosen to coincide with periods when the drillstring is stationary, such as when drillstring connections are made at every 30 to 90 feet or so. In some circumstances, however, the toolface offset can change dramatically over the course of a few feet drilled, and the need to calculate the toolface offset while the drilling assembly is stationary is impractical. Accordingly, the borehole in such circumstances can proceed in the wrong direction.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.